Using "developer command prompt" coerces NRE from docfx

[dovholuknf]

Operation System: Windows 10

DocFX Version Used: 2.45.1.0

Template used: the walkthrough - https://dotnet.github.io/docfx/tutorial/walkthrough/artifacts/walkthrough2.zip

Steps to Reproduce:

1. download and unzip https://dotnet.github.io/docfx/tutorial/walkthrough/artifacts/walkthrough2.zip to c:\temp\docfx
2. cd to C:\temp\docfx\walkthrough2
3. run docfx (inside a "Developer Command Prompt for VS 2017" OR "Developer Command Prompt for VS 2019")

Expected Behavior:
It should work

Actual Behavior:
Fails:

C:\temp\docfx\walkthrough2>docfx
[19-10-02 04:37:15.229]Info:Config file docfx.json found, start generating metadata...
[19-10-02 04:37:15.348]Info:[MetadataCommand.ExtractMetadata]Environment variable VSINSTALLDIR is set to C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\, it is used as the inner compiler.
[19-10-02 04:37:15.537]Info:[MetadataCommand.ExtractMetadata]Loading projects...
[19-10-02 04:37:15.957]Warning:[MetadataCommand.ExtractMetadata](C:/temp/docfx/walkthrough2/src/HelloDocfx1/HelloDocfx1.csproj)Error opening project C:/temp/docfx/walkthrough2/src/HelloDocfx1/HelloDocfx1.csproj: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.. Ignored.
[19-10-02 04:37:15.972]Info:[MetadataCommand]Completed Scope:MetadataCommand in 629.3258 milliseconds.
[19-10-02 04:37:15.973]Error:Error extracting metadata for C:/temp/docfx/walkthrough2/src/HelloDocfx1/HelloDocfx1.csproj: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.<SaveAllMembersFromCacheAsync>d__13.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.<ExtractMetadataAsync>d__11.MoveNext()
[19-10-02 04:37:15.975]Info:Completed in 636.251 milliseconds


Build failed.
[19-10-02 04:37:15.982]Warning:[MetadataCommand.ExtractMetadata](C:/temp/docfx/walkthrough2/src/HelloDocfx1/HelloDocfx1.csproj)Error opening project C:/temp/docfx/walkthrough2/src/HelloDocfx1/HelloDocfx1.csproj: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.. Ignored.
[19-10-02 04:37:15.985]Error:Error extracting metadata for C:/temp/docfx/walkthrough2/src/HelloDocfx1/HelloDocfx1.csproj: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.<SaveAllMembersFromCacheAsync>d__13.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.<ExtractMetadataAsync>d__11.MoveNext()
        1 Warning(s)
        1 Error(s)

Additional Comment
Also note that if you do this using a 'plain' command prompt docfx will succeed. Something must be on the path that's causing trouble

[superyyrrzz]

Root cause should be #4782 . Close and keep track there.